openwrt ipv6

dhcp.lan.ra_default='1' ？？ 这是什么意思？？
参考：https://openwrt.org/docs/guide-user/base-system/dhcp
Default router lifetime in the RA message will be set if default route is present and a global IPv6 address (0) or if default route is present but no global IPv6 address (1) or neither of both conditions (2)